Optical retro-reflective apparatus with modulation capability

ABSTRACT

An optical MEMS retro-reflective apparatus with modulation capability having a retro-reflecting structure including a pair of reflective surfaces; and a MEMS device for moving at least one of the reflective surfaces of said pair of reflective surfaces relative to another one of the reflective surfaces of said pair of reflective surfaces a distance which causes the pair of reflective surfaces to switch between a reflective mode of operation and a transmissive mode of operation. A substrate and a moveable grating structure may be substituted for the reflective surfaces.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. Provisional Patent Application No.60/420,177, filed Oct. 21, 2002, and entitled “Conformal Retro-Modulatoror Optical Devices,” the disclosure of which is hereby incorporatedherein by reference. The subject matter of the present application mayalso be related to the following U.S. Patent Application: “ConformalRetro-Modulator Optical Devices” Ser. No. 10/690,486, filed Oct. 20,2003.

TECHNICAL FIELD

This invention relates to an optical retro-reflector device withmodulation capability preferably in the kHz to MHz range (or evengreater). This device is compact and rugged and can be used in a varietyof communication, interconnect, and remote sensing applications.Clusters of several retro devices on a common platform can be used toaddress a large field of view. A similar cluster can also be used toprovide for additional wavefront correction beyond tilt-errorcompensation (i.e., wavefront errors of odd order in phase), forming aso-called “pseudo-conjugator.” The retro-modulator can be functional invisible, IR, or mid-IR portions of the spectrum, depending on thematerials and coatings employed. The disclosed devices preferablyutilize MEMS-based technology to alter the retro-reflective nature ofthe disclosed devices.

BACKGROUND OF THE INVENTION

A retro-modulation device with modulation capability, as disclosedherein, includes a compact, monolithic optical MEMS structure, whichuses mechanical motion, induced electrostatically or thermally, todisplace various components of the structure for the modulationencoding. In general, retroreflectors have the property of taking anincident optical (or RF) beam received from a source and redirecting thebeam back to the source via a sequence of reflections within theretro-device As such, retro-reflective devices, in essence, “track” thesource location and thus provide auto-compensation for beam wander,relative platform motion and wobble. In addition, by encoding amodulation signal at (or within) the retro-reflective device, the returnbeam can be encoded with the information from the modulation signal. Inthis manner, information can be directed from a remote site back to thelocation of an interrogation or source beacon without the need for anoptical (or RF) source or transmitter at the remote site.Retro-reflection and modulation devices have myriad applications tofree-space links, optical interconnects, remote sensors, IFF/IDbattlefield scenarios, guidance control for free-space or underwaterplatforms, etc.

The retro-reflecting structures can be in the form of a corner cube orcat's eye configuration as well as an array of such devices (the latteris referred to as a pseudo-conjugator). In either case, in accordancewith the present invention, a modulation signal is used to displace theelement(s) of the structure to, in essence, reversibly “defeat” theretro-directive capability of the device. The result is that the returnbeam returns back to the transceiver location as an amplitude-modulatedbeam or as an on-off digitally or binary modulated beam, bearing thedesired encoded temporal information. Since the mechanical displacementis cyclical (billions of cycles have been demonstrated using opticalMEMS devices in general), modulation information can be relayed usingsuch a device with high reliability over a very long period of time.Modulation bandwidth in the range of kHz to many MHz are possible usingthis scheme.

Large arrays of retro-reflecting structures can be utilized as disclosedin U.S. provisional patent application Ser. No. 60/420,177 noted above.

Both types of retro-reflection devices (cat's eye and corner cubestructures) are known, per se, in the prior art. Additionally, the fourvariations of optical switches subsequently discussed herein have alsobeen discussed in the prior art. The present invention merges the twonotions (retro-reflection devices with optical switching methods) toform a retro-modulator. This concept is not apparent to persons skilledin the art, since the primary applications of the four classes ofswitches have been presented by experts in the field in the context ofhigh-definition display-mode devices (spatial light modulators, SLMs),high-density optical interconnection elements for telecommunicationsnetworks, wavelength divisional multiplexing (WDM) optical devices ofhigh-density communication traffic networks and optical spectrometers.The notion of combining any of these switching approaches for use asmodulated retro-devices is not known in the prior art. Therefore, thepresent invention is novel and, as discussed above and below, has myriaduses and applications in a variety of fields and scenarios. Thedisclosed retro-modulator should find itself very useful in manyapplications. For example, the disclosed retro-modulators can be usedfor combat ID/IFF, remote sensor optical links, interconnects,free-space links to communicate with missiles, high-speed smartprojectiles, underwater devices, UAVs, and micro-UAVs, etc., and forcommunication links in general (telecommunication, wireless,interconnects, etc.). The disclosed retro-modulators can be used inconnection with inter vehicle communication and accident avoidancetagging.

One object of this invention is to make it possible to realize a compactremote communication device, with modulation capability. The advantagesof this invention include:

-   -   1. The disclosed retro-modulation devices are preferably        passive; therefore the optical interrogation source needs only        to be located at the interrogation/beacon site, and not        necessarily at the retro-device location.    -   2. The disclosed retro-modulations device can be of rugged,        lightweight, compact, low-power consuming, and low cost        construction, yet it can operate at elevated temperatures.    -   3. MEMS devices, if used, can function at high-g values owing to        their small sizes and mass and their favorable mass to size        ratio.    -   4. The retro nature of the device provides for automatic tilt        error correction as well as automatic beam wander compensation        (e.g., wobbling, relative platform motion).    -   5. The disclosed retro-modulation devices may be made very        compactly using MEMS devices, for example, in combination with a        variety of photonics components, such as detectors, optical        elements, modulators, fibers and other integrated optical        components, etc., with which the MEMS devices may be integrated        and interfaced.    -   6. The disclosed retro-modulation devices can be fabricated        using a variety of well known materials, such as Si, poly-Si,        GaAs, InP, etc., and can be manufactured in high volume using        commercially available technology.

Prior art includes conventional retro-reflectors with upstream externalmodulators (e.g., liquid crystals, MQWs, E-O crystals). In addition, theprior art includes phase-conjugate mirrors with modulation capability(via applied electric fields or modulated pump beams). The prior artalso describes optical MEMS with a modulation capability.

It has been known in the prior art that a MEMS device can be associatedwith a retro-reflecting corner cube. By tilting one of the mirrors ofthe corner cube with desired modulation information, the retro-returnproperty of the device can be controlled in a time-wise fashion. Thatis, the retro-aspect of the device can be defeated temporally (i.e., thereflected light no longer returns along the reverse direction from whichthe corner cube was illuminated). This results in apparent modulation ofthe light received back at the source location, since the light eitherreturns back to its source location or is diverted over another path,according to the tilt of one of the mirrors in the corner cube device. Adrawback is that a relatively large angular displacement of the cornercube mirror is necessary to divert the backward-propagating light beyondthe diffraction spread of the small MEMS device. Assuming a 50 μm scalesize, a 2° tilt is required to divert the reflected light away from itsdiffraction-limited return path to the sender. This implies a 2 μmdisplacement of the mirror edge, which is relatively large, and placesdemands on the device geometry, the drive voltage, its slew rate, etc.,of the MEMS device controlling the mirror.

The present invention improves upon this existing art by imposing themodulation via a diffractive effect (or a displacement of a very smallcantilever) or via an optical Fabry-Perot effect. Each effect can beutilized in either a cat's eye or a corner cube arrangement. In thedisclosed embodiments, the required mechanical displacement can be assmall as 100 nm, or only 5% of what the prior art requires. This impliesthat the required drive voltage or electrical drive current for thepresent invention would be only 5% of that required for the prior artdevices, thereby reducing the drive power by the square of this ratio,potentially increasing the bandwidth of the modulation from the KHz toMHz. The required mechanical displacement is a small fraction of thewavelength of the light modulated by the disclosed devices.

Also, the present invention can lead to a larger depth-of-modulation fora given drive voltage. This follows since the beam in the “off-state” isdeflected over a much greater angle for a given MEMS displacement(using, e.g., the diffraction-based embodiments of this invention). Thesame argument also applies to the Fabry-Perot embodiments. In bothcases, this reduced MEMS displacement also enables the required drivevoltage to be even less for the same depth-of-modulation performance,relative to the mirror-tilt-based corner cube (the prior art). In yet athird embodiment of this invention, a small MEMS cantilever is employedat the focal plane of a lens to encode the modulation information. Inthis case, the MEMS device can be much smaller than the tilt device inthe corner cube geometry (e.g., 10 μm versus about 50 to 100 μm),resulting in a lower voltage and torque required for the modulationencoding. The prior art also includes O-MEMS for displays (TI's DMD SLMsusing MEMS cantilevers; Silicon Light Machine's diffractive-basedstructures), and tunable MEMS optical filters for WDM. These devicespertain to large-screen, high-definition TV multi-pixel display systemsor to add/drop devices, but not to retro-communication or remote sensingdevices. The prior art does not discuss or imply the possibility ofusing these structures as elements for retro-devices or asretro-modulation devices, nor does the prior art imply this applicationor even imply how they can be designed or used for the devices disclosedherein.

Examples of prior art devices are shown in FIGS. 1, 2 a and 2 b. FIG. 1shows a passive retro-reflector device 24 with an external modulator 22.The construction of the retro-device can be in the form of an embossedmold with a reflective coating (in the mid-IR range), a corner cube, athree-mirror structure, lens/mirror combination, or an array of thesame. A corner cube is depicted in FIG. 1 for ease of illustration. Theexternal modulator 22 can be in the form of an electro-optic amplitudeor phase modulator (e.g., a liquid crystal) or a multi-quantum wellelectro-absorptive modulator.

A laser 10 forms a laser beam 12 which is directed to a communicationsretro-reflector 20. Reflector 20 houses the aforementioned passiveretro-reflector device 24 and its associated external modulator 22. Beam12 usually will have to transit a propagation path 14, as will a beam 12r reflected by retro-reflector 20. The reflected beam 12 r is modulatedwith data by means of modulator 22. The modulated data is detected by adetector 18 and a beam splitter 18 may be conveniently used to separatethe reflected beam 12 r from the incident beam 12.

FIGS. 2 a and 2 b show additional prior art devices. In FIG. 2 a, theretro-reflector device 24 is a monolithically fabricated optical MEMSstructure using reflective elements to form the required device. In thiscase, the mirrors of a corner cube can be deflected (by tilting them onan axis 27) to defeat the retro-directive property of the corner cube 24(effectively resulting in a modulated return beam). In FIG. 2 a, a MEMSdevice 26 is used to move at least one of the mirrors (or otherreflector element) 24 a of a corner cube reflector 24. The MEMS deviceis responsive to a signal on line 20 s for controlling the actuation ofthe MEMS device 26. By putting data on line 20 s, the reflected signalis, in effect, modulated since a detector 18 would only “see” thereflected beam 12 r when mirror 24 a is in its “normal” position. InFIG. 2 a dashed lines are used to illustrate movement of mirror 24 afrom is “normal” (solid line) portion to its actuated (dashed line)position. When mirror 24 a is in its actuated position, the beam isdeflected in a direction 12 d which does not permit detection bydetector 18. As discussed above, the relatively large size of thisMEMS-activated mirror 24 a, coupled with the constraint of having todeflect the beam over an angle d in excess of the diffraction-limitedspread of the retro-directed beam, limits the modulation bandwidth for agiven drive voltage and slew rate. This constraint is greatly relaxedusing the embodiments of the present invention.

Another example of the prior art is shown in FIG. 2 b, where aphase-conjugate mirror 25 is shown with an applied modulation capability(the modulation can be externally applied as in the case of FIG. 1). Theconjugator wavefront reverses the incident beam 12 and produces areflected beam 12 r, while, at the same time, the modulator encodestemporal information onto this beam.

DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a conventional retro-modulator with external modulationelement;

FIG. 2 a depicts a conventional MEMS corner cube with mirror deflector;

FIG. 2 b depicts a conventional phase-conjugate mirror with modulationcapability;

FIG. 3 shows a cat's eye retro-reflector with compact MEMS cantilever toprovide modulation encoding;

FIG. 4 depicts a cat's eye retro-modulator using a binary, switchablegrating element for encoding;

FIGS. 4 a and 4 b show the switchable grating element used in theembodiment of FIGS. 4 and 6 in greater detail and in two positions ofdiffraction and specular reflection, respectively;

FIG. 5 shows a cat's eye retro-modulator using a MEMS binary switchableFabry-Perot element for encoding;

FIGS. 5 a and 5 b show the switchable Fabry-Perot element used in theembodiment of FIG. 5 in greater detail and in two positions oftransmission and specular reflection, respectively;

FIG. 6 illustrates a corner cube retro-modulator using a binary MEMSswitchable grating (see FIGS. 4( a) and 4(b) for a more detailed view ofthe grating) for encoding;

FIG. 7 depicts a corner cube retro-modulator using a binary MEMSFabry-Perot element (see FIGS. 5( a) and 5(b) for a more detailed viewof the MEMS Fabry-Perot element) for encoding;

FIG. 8 depicts an embodiment of a cat's eye retro-reflector whichutilizes a moveable bubble in a TIR (total internal reflection) device;and

FIG. 9 depicts an embodiment of a corner cube retro-reflector whichutilizes a moveable bubble in a TIR device.

FIG. 10 depicts an embodiment of a corner cube retro-reflector withthree mirrored surfaces each disposed orthogonally with respect to eachother, with one of the mirrored surfaces being an electricallycontrollable reflective surface;

FIG. 10 a depicts an embodiment of a corner cube retro-reflector withthree mirrored surfaces each disposed orthogonally with respect to eachother, with more than one of the mirrored surfaces being an electricallycontrollable reflective surface;

FIG. 10 b depicts an embodiment of a corner cube retro-reflector withthree mirrored surfaces each disposed orthogonally with respect to eachother, with one of the mirrored surfaces being an electricallycontrollable pixelated reflective surface;

FIGS. 10 b-1 is a more detailed view of a pixelated, reflective surfaceof the embodiments of FIGS. 10 b or 10 c, and shows the preferredorientation variations and grating period variations for different typesof pixelated reflective surfaces;

FIG. 10 c depicts an embodiment of a corner cube retro-reflector withthree mirrored surfaces each disposed orthogonally with respect to eachother, with one of the mirrored surfaces being an electricallycontrollable pixelated reflective surface and the other mirroredsurfaces being electrically controllable; and

FIG. 10 c-1 depicts piston actuated mirrors which may be used with theembodiments of FIGS. 10 b or 10 c.

DETAILED DESCRIPTION

Various embodiments of the present invention are shown in FIGS. 3, 4, 5,6, 7, 8, 9, 10, 10 a, 10 b, 10 b-1 and 10 c. In these figures,retro-modulators using cat's eye as well as corner cube structures asthe basic retroreflective configurations are shown. The (binary)modulation can be encoded using four different methods, which can beintegrated into these two (cat's eye and corner cube) basicretroreflective structures: (1) deflection of the optical beam usingcompact MEMS cantilevers; (2) diffraction of the beam using switchablegrating structures; (3) reflection of the beam using binary switchableFabry-Perot elements; (4) switching of an optical beam using a moveablebubble in a TIR (total internal reflection) binary set of states.

FIGS. 3, 4, 5 and 8 show examples of embodiments using cat's eyedevices. FIG. 3 shows an embodiment using a cat's eye retro-device witha small MEMS cantilever device 26 placed at the backplane 34 of a lensor diffractive element 32. The cantilever device 26 can be driven(electrostatically, optically or thermally) to deflect the beam into oneof two different angular states by moving the back plane 34 to location34 d, so that the return beam can be effectively modulated. Typically,the MEMS device 26 is driven by an electrical or an optical signal online 20 s. Without a signal, a “normal” retro-direction is realized (asindicated by the presence of reflected beam 12 r) and in the presence ofthe drive voltage (in the case of a non-optical MEMS device embodiment),the retro-direction 12 r is defeated and the non-retro direction 12 d isinstead realized. Of course, the sense of the two states can beinverted, if desired. Such cantilevers 26 can be relatively small (10 μmscale size) so that operation at high modulation rates (kHz and above)can be realized and low modulation powers need be applied. Large arraysof these basic cantilevers (all in a plane, and without a retro-device)have been employed for spatial light modulators for display modeapplications.

FIG. 4 shows an alternate embodiment using a cat's eye retroreflector,but, now with a binary, switchable grating structure 44 shown in greaterdetail in FIGS. 4 a and 4 b. With the signal or voltage off (on line 20s), the grating 44 b is in the plane of the substrate 44 a, so that aspecular reflection is realized in direction 12 r (in FIGS. 4 a and 4 blens 32 is omitted for ease of illustration); but with an applied signalor voltage on line 20 s, the grating 44 b now is above (in a directiontowards the source of the incident beam 12 as shown by FIG. 4 b) thesurface and diffracts the incident beam 12 over an angle beyond theretro-direction to the non-retro direction 12 d. Note that the requireddisplacement of the grating 44 b from the substrate 44 a need only be inthe range of λ/4 (where λ is a wavelength of the incident beam 12), sothat small drive signals and drive powers are required for the MEMSdevice 26 to effect the needed movement and thereby allow modulation tobe applied to beam 12 (on beam 12 r) by device 20. Large arrays of thesebasic gratings (all in a plane, and without a retro-device) have beenemployed for spatial light modulators for display mode applications.

FIG. 5 shows a cat's eye device with a switchable MEMS-based opticalfilter 54 to provide the modulation. A two-mirror Fabry-Perot filter 54is configured using a pair of compact reflecting elements 54 a, 54 b sothat in one state, it reflects the desired beam 12 r, whereas in theother state, the beam is transmitted through the optical cavity as beam12 d. One or both of the compact reflecting elements 54 a, 54 b ispositioned by at least one MEMS device 26. The two-mirror Fabry-Perotfilter 54, which is known per se in the prior art, is shown in greaterdetail by FIGS. 5 a and 5 b. By displacing the pair of MEMS reflectingelements from a spacing L₁ (see FIG. 5 a) to a spacing L₂ (see FIG. 5 b)between reflecting elements 54 a, 54 b, the beam can be switched.Displacements as small as 100 nm (0.1 micron) have been demonstrated forWDM applications, resulting in a very small drive voltage relative totypical (microns) MEMS-based displacement devices. The system 20 isconfigured so that the reflected beam 12 r completes the retro-mode,whereas the transmitted beam 12 d does not. This device 20 may not havea limited field of view, dictated by the angular dependence of theFabry-Perot cavity spectral response.

FIG. 8 depicts a TIR device 74 arranged as a cat's eye retro reflector.The TIR device 74 includes a moveable bubble 74 f placed between a pairof glass or Si substrates 74 a & 74 b, forming a three-elementstructure, as is shown by FIG. 8. This bubble 74 f, when positioned(thermally, electrostatically, etc.) along the path of the optical beam12, enables transmission of the beam through the three-element structure74 a, 74 f, 74 b since it preferably has an index of refraction the sameas that of the substrates 74 a, 74 b. When the bubble is positioned outof the path of beam 12, the incident beam 12 experiences total internalreflection (TIR), and is scattered onto many different directions.Substrate 74 b has a conductive and reflective surface 74 c on its rearsurface for reflecting beam 12 after it passes through bubble 74 f.

The bubble can be controlled (moved) by imparting electrical signalsbetween transparent electrodes 74 d on the surface of substrate 74 a andthe conductive surface 74 c. An electric field between one of theelectrodes 74 d and surface 74 c will attract bubble 74 f.

The internally facing surfaces 74 e of substrates 74 a and 74 b arepreferably etched to cause the total internal reflection (TIR) to occurwhen the bubble 74 f is not in way of the beam 12. However, when bubble74 f has been urged to fall within the path of beam 12, it effectivelywets those portions of the etched surfaces 74 e in way of the beam 12allowing the beam to pass easily through the bubble 74 f and reflect offsurface 74 c.

This binary switching device 74 can be incorporated into either thecat's eye device, as shown in FIG. 8, or, as will be seen, into thecorner cube retro-reflector as shown by FIG. 9.

FIGS. 6, 7, 9 and 10 show embodiments using corner cubes as the basicretro-reflecting device. FIG. 6 depicts an embodiment employing a cornercube retro-reflector 24 a which has one mirror 28 of a normal cornercube reflector replaced with a switchable grating structure 44 (seeFIGS. 4 a and 4 b). The device 24 a is configured so that with thegrating 44 in an off state, a specular return is realized on path 12 rvia mirror 28, thereby providing the corner cube's retro capability.With an applied signal voltage on line 20 s, the grating now diffractsthe incident beam 12 preferably beyond the diffraction-limitedretro-angle in a direction 12 d. The grating structure 44 is used as adiffractive element (i.e., effectively a reflector), which can be raisedor lowered (in-plane or out-of-plane) to deflect or not deflect theincident beam in the retro-direction 12 r, resulting in an effectivebinary modulation of the retro-reflected beam 12 r. Note that a verysmall displacement (λ/4) is required, as compared with a conventionalmirror element for a MEMS controlled corner cube, whose requireddeflection may be a factor of 5 or more times greater than thatnecessary by the embodiments using MEMS devices disclosed herein.

FIG. 7 shows a corner cube embodiment using a switchable Fabry-Perotfilter 54 as the binary-encoded modulation element (see FIGS. 5 a and 5b). The Fabry-Perot filter 54 has MEMS 26 controlled mirrors 54 a, 54 b.Here, the reflected state of the filter 54 is used to realize aretro-return beam 12 r via mirror 28, whereas the transmitted stateprovides for the off state by passing the incident beam 12 as beam 12 d.

A detector 60 (see FIGS. 5 and 7) can be provided to easily detect thepresence of incident beam 12, which is useful in a communicationsapplication of the present invention such as that shown by U.S.provisional patent application Ser. No. 60/420,177 mentioned above. Insuch an embodiment, the Fabry-Perot devices of FIG. 5 or 7 could besubstituted for the Fabry-Perot Multiple Quantum Well devices of U.S.provisional patent application Ser. No. 60/420,177 mentioned above, forexample.

The detector 60 can also detect information encoded on the incident beam12. The encoded information, in addition to data, can include a highfrequency jitter on the data. When detector 60 is in a detection mode,then the Fabry-Perot device 54 is passing incident signal 12 as opposedto absorbing incident signal 12. The jitter can be used to test how wellthe Fabry-Perot device is passing the incident signal since the highfrequency jitter can be detected and decoded to determine whether theFabry-Perot device 54 is optimally tuned or whether it is slightly offfrequency. For example, the plates 54 a and 54 b could be 1) properlyspaced, 2) too close or 3) too far apart. The analog signal output bydetector 60 can be analyzed by a control system 62 to determine whetherthe plates are properly spaced. If not, then a DC control voltage outputby the control system 62 can be applied via an operational amplifier 64to the MEMS device 26 to adjust (increase or decrease) the spacing ofplates 54 a, 54 b appropriately. The data signal on line 20 s becomesactive whenever the device is used to transmit data on the return beam12 r. The received data is passed via a low pass filter 68 to a computer72 for decoding the incident data and for generating the transmitteddata on 20 s.

The light 12 d passing through to the detector 70 can be utilized toadjust (fine tune) the Fabry-Perot device 54 to compensate for:

-   -   (1) wavelength changes in beam 12;    -   (2) thermal effects (expansion, etc of the system);    -   (3) light arriving off the axis of beam 12 (see the off-axis        beam 12 oa). The Fabry-Perot device 54 has a wide field of view,        so it can “see” beam 12 oa, but its plates 54 a, 54 b might then        be improperly spaced for beam 12 oa since the light of beam 12        oa will travel farther within device 54 than would the light of        beam 12.

The control system 62 can be used to help compensate for such issues bydetecting the high frequency jitter. If control system is not used, thenthe Fabry-Perot device may be slightly mistuned, thereby decreasing itssensitivity somewhat.

In the case of the embodiments of FIGS. 3-9, the reflected beam 12 r isgenerally co-incident with the incident beam 12 and would normally beseparated therefrom, for detection purposes, at a location remote fromapparatus 20 using a beam splitter 16 (previously discussed withreference to FIG. 1) and the split-off beam would then be detected usinga detector 18 (also previously discussed with reference to FIG. 1).

The aforementioned corner cube embodiments of FIGS. 6, 7 and 9 have allbeen depicted with preferably two planar surfaces, namely a mirroredsurface 28 and a surface 44, 54 or 74 whose reflection abilities can bealtered in response to an electrical signal 20 s. Surface 44, 54 or 74will hereinafter be referred to generically as an electricallycontrollable reflective surface. Additionally, corner cube devices canhave two or three nominally reflective surfaces and, moreover, one, twoor all such surfaces can be implemented as an electrically controllablereflective surface whose reflection abilities can be altered in responseto an electrical signal 20 s.

FIG. 10 depicts a corner cube with three mirrored surfaces each disposedorthogonally with respect to each of the other two mirrored surfaces. Inthis embodiment, two surfaces 28 are preferably simple mirrors while onesurface is an electrically controllable reflective surface 84. Theelectrically controllable reflective surface 84 is depicted withparallel lines to reflect the fact that this surface may well havegratings associated therewith, as shown in FIGS. 4 a, 4 b, and 8.

FIG. 10 a is similar to FIG. 10, but in this embodiment all threemirrored surfaces are implemented as electrically controllablereflective surfaces 84. In this embodiment the electrically controllablereflective surfaces are labeled 84 a, 84 b and 84 c. Of course, surfaces84 a, 84 b and 84 c are each disposed orthogonally with respect to eachof the other two surfaces. The spacings of the parallel lines differ foreach of the surfaces 84 a, 84 b and 84 c to reflect the fact that thespacings or intervals of their gratings 44, if used, need not be thesame for each surface 84 a, 84 b and 84 c. Indeed, it is preferable thatsuch spacings or intervals be different as that should help break up adeflected beam 12 d and help make it even less detectable.

FIG. 10 b is again similar to FIG. 10, but in this case one of themirrored surfaces is implemented by an electrically controllablereflective surface 84 which is pixelated, as is represented by the gridpattern shown on surface 84 of FIG. 10 b. A pixelated electricallycontrollable reflective surface 84 can be implemented as a grid or arrayof pixelated electrically controllable reflective elements 94. Eachelement 94 can be implemented as a MEMS controlled device of the type,for example, previously discussed with reference to FIGS. 2 a, 4 or 5 orby a MEMS element whose moveable arm is coupled to a small mirroredsurface 96 (see FIG. 10 c-1). If the individual elements 94 areimplemented as moveable gratings as shown in FIGS. 4, 4 a and 4 b, thenthe gratings 44 of neighboring elements 94 in pixelated surface 84 arepreferably disposed at angles relative to each other as shown in themore detailed view of FIG. 10 b-1 and are preferably implemented withdifferent grating periods.

If the individual elements 94 are implemented with small mirrors thatcan be tilted as shown in FIG. 2 a, the tilt axes 27 for neighboringelements 94 are preferably disposed at angles and adjacent differentedges of the neighboring elements 94, again as represented in FIGS. 10b-1.

Alternatively, a pixelated electrically controllable reflective surface84 can be implemented by a spatial light modulator (SLM), whichfunctions as a spatial phase modulator in a monolithic package.

FIG. 10 c is similar to FIG. 10 b, but in this embodiment the remainingtwo mirror surfaces of the embodiment of FIG. 10 b are here depicted aselectrically controllable reflective surfaces 84 which have associatedgratings or may also be pixelated (as represented by the dashed lines onone of the three surfaces 84). The incident beam 12, the return beam 12r and the deflected beam(s) 12 d are also depicted in this Figure.

The spacings (periods) of the gratings preferably fall in the range ofabout 1-100 μm. Due to the Bragg condition, the smaller the gratingperiod, the larger the angle by which a reflected beam is reflected awayfrom its usual (for a planar surface) angle of reflection. However, thecorner reflector will also have some dispersion associated with itsreflected beams, including beams 12 r and 12 d, due to aperture effects.The deflected beam(s) 12 d should preferably be deflected by a greaterangle from beam 12 r than that which would be attributable to normalaperture effects so that an interrogating beam 12 sees little or noreflection(s) during a portion of the time the beam is being modulated(for example when the amplitude of the modulating signal is at itsmaximum (or minimum)) to increase the contrast ratio between beams 12 rand 12 d as the incoming light is modulated by the corner cube device.

FIG. 10 c-1 depicts mirrors 96 which are moved in a piston-like fashionrelative to neighboring mirrors 96 (keeping the associated mirrors inparallel planes as opposed to tilted as done when the embodiment of FIG.2 a is used with the embodiment of 10 b). The individual mirrors 96 aremoved by MEMS devices 26 and when the MEMS devices 26 are unactuated,the mirrors preferably align in a planar configuration 98. The size ofthe piston-like mirrors 96 are also preferably in the range of 1-100 μmon a side for the same reasons as given above. If tilting mirrors areused, then they can each fill or occupy one of the elements 84 in thearray.

In FIG. 10 c-1 only four mirrors 96 are shown for ease of illustration,it being understood that the mirrors 96 would preferably be arranged ina large two dimensional array of mirrors 96.

Generally speaking, the larger the number of elements 84 in an array,the better the array is in causing a greater number of deflected beams12 d to be generated during modulation, but with a downside ofincreasing the cost of the corner cube device due to the fact that thenumber of MEMS devices will similarly increase (assuming that there isone MEMS device, for example, associated with each pixel 84). In amilitary application, for example, it may well be that one does not wantsome undesirable third party trying to read the deflected beam 12 d. Bythrowing the deflected beams 12 d in thousands (for example) ofdifferent directions, the amount of light headed in any particulardirection that is off the normal retro-reflection axis of the returnbeam 12 r is reduced considerably, thereby making it more difficult foran undesirable third party to try to eavesdrop on the deflected beam(s)12 d.

The MEMS devices discussed above are described as being electricallycontrolled, which is the conventional manner for controlling MEMSdevices. However, optically controlled MEMS devices are known in the artand the MEMS devices used in the disclosed embodiment could be eitherelectrically or optically controlled. When array(s) of pixelatedelectrically controllable reflective elements are utilized, then theindividual reflective elements in the array can be controlled either bya common control signal or by individual control signals.

Having described this invention in connection with a number ofembodiments, modification will now certainly suggest itself to thoseskilled in the art. As such, the invention is not to be limited to thedisclosed embodiments except as required by the appended claims.

1. An optical retro-reflective apparatus with modulation capabilitycomprising: a retro-reflecting Fabry-Perot structure including a pair ofreflective surfaces; and a micromechanical device for moving at leastone of the reflective surfaces of said pair of reflective surfacesrelative to another one of the reflective surfaces of said pair ofreflective surfaces a distance which causes the pair of the reflectivesurfaces to switch between a reflective mode of operation and atransmissive mode of operation.
 2. The apparatus of claim 1 wherein theretro-reflecting structure includes a corner cube arrangement with thepair of reflective surfaces forming at least one angled reflectingsurface of the corner cube arrangement and another reflecting surfaceforming another angled reflecting surface of the corner cubearrangement.
 3. The apparatus of claim 2 further including a detectordisposed to receive an optical beam passing through the reflectivesurfaces when in said transmissive mode of operation.
 4. The apparatusof claim 2 further including a detector disposed to receive an opticalbeam reflected from the reflective surfaces when the reflective surfacesare in said reflective mode of operation.
 5. The apparatus of claim 1wherein the micromechanical device is a MEM device made usingphotolithographic techniques.
 6. An optical retro-reflective apparatusfor modulating an optical beam, the apparatus comprising: aretro-reflecting structure including a substrate and a moveable gratingstructure; and a micromechanical device for moving the moveable gratingstructure relative to the substrate to cause the retro-reflectingstructure to switch between a retro-reflective mode of operation and anon-retro-reflective mode of operation, the micromechanical device beingresponsive to a signal to impart modulation to an optical beam which isretro-reflected from the retro-reflecting structure.
 7. The apparatus ofclaim 6 wherein the retro-reflecting structure includes a corner cubearrangement with said substrate and moveable grating structure formingat least a portion of one reflecting surface of the corner cubearrangement and at least another reflecting surface forming anotherreflecting surface of the corner cube arrangement.
 8. The apparatus ofclaim 7 further including a detector disposed to receive a beamreflected from the substrate and moveable grating structure when thesubstrate and moveable grating structure are in said reflective mode ofoperation.
 9. The apparatus of claim 7 wherein said one reflectingsurface of said corner cube arrangement is pixelated by a plurality ofmoveable grating structures.
 10. The apparatus of claim 9 wherein thegratings of one moveable grating structure of said plurality of moveablegrating structures is rotated about a central axis thereof related toneighboring moveable grating structures.
 11. The apparatus of claim 10wherein the at least another reflecting surface has a moveable gratingstructure associated therewith which is responsive to said signal forimparting modulation to the optical beam that is retro-reflected fromthe retro-reflecting structure.
 12. An apparatus for retro-reflectingand modulating an optical beam comprising: a. a retro-reflectingstructure having at least one moveable optical element for selectivelyreflecting the optical beam impinging the retro-reflecting structure,the moveable optical element having a first position in which theretro-reflecting structure retro-reflects the optical beam and having asecond position in which the retro-reflecting structure does notretro-reflect the optical beam, the first and second positions beingspaced by a distance less than a wavelength of the optical beam; and b.a micromechanical device for moving said at least one moveable opticalelement in response to a modulation signal to thereby modulate theoptical beam as a modulated retro-reflected beam.
 13. The apparatus ofclaim 12 wherein the retro-reflecting structure includes at least a pairof reflective surfaces, at least one of said surfaces including the atleast one optical element which is moved less than a wavelength of theoptical beam in order to modulate the retro-reflected beam.
 14. Theapparatus of claim 13 wherein the pair of reflective surfaces arearranged in either a cat's eye or a corner cube configuration.
 15. Theapparatus of claim 12 wherein the retro-reflecting structure includes asubstrate and a grating structure, at least one of said substrate andsaid grating structure comprising the at least one optical element whichis moved less than a wavelength of the optical beam in order to modulatethe retro-reflected beam.
 16. The apparatus of claim 15 wherein thesubstrate and grating structure are arranged in either a cat's eye or acorner cube configuration.
 17. An optical retro-reflective apparatuswith modulation capability comprising: a first reflective surface; asecond reflective surface having a first position in which theretro-reflecting apparatus retro-reflects an optical beam and having asecond position in which the retro-reflecting apparatus does notretro-reflect the optical beam; and a micromechanical device operable tomove the second reflective surface between the first position and thesecond position, wherein the first reflective surface and the secondreflective surface are parallel to each other in the first position andthe second position.
 18. The apparatus of claim 17 wherein the first andsecond positions being spaced by a distance less than a wavelength ofthe optical beam.
 19. An optical retro-reflective apparatus formodulating an optical beam, the apparatus comprising: a retro-reflectingstructure including a substrate and a moveable grating structure havinga first position in which the retro-reflecting structure retro-reflectsan optical beam and having a second position in which theretro-reflecting structure does not retro-reflect the optical beam; anda micromechanical device for moving the moveable grating structurebetween the first position and the second position, wherein the moveablegrating structure and the substrate are parallel to each other in thefirst position and the second position.
 20. The apparatus of claim 6,wherein the substrate is at least partially reflective.
 21. Theapparatus of claim 6, further comprising a partially reflective surface.22. The apparatus of claim 21 wherein the moveable grating structure isconfigured to at least partially reflect an optical beam towards thepartially reflective surface.
 23. The apparatus of claim 19, furthercomprising a partially reflective surface.
 24. The apparatus of claim 23wherein the moveable grating structure is configured to retro-reflect anoptical beam towards the partially reflective surface when in the firstposition.
 25. The apparatus of claim 12 wherein the retro-reflectingstructure includes a first grating structure and a second gratingstructure, at least one of said grating structures comprises the atleast one optical element which is moved less than a wavelength of theoptical beam in order to modulate the retro-reflected beam.